The presence of neuropeptides as well as peptide hormones in lung has been well established. Previously we have detected met5-enkephalin-arg6-phe7 (ME-arg-phe), bombesin and substance P-like peptides in the rat lung by specific radioimmunoassays. In this study, the cellular locations of these peptides were investigated by immunohistochemical technique. Bombesin and substance P immunoactivities were found in nerve fibers present in bronchi and blood vessels while ME-arg-phe immunoactivity was localized in small APUD-like cells around the wall of some bronchi. Because of its cellular location, the release of ME-arg-phe from lung slices was studied. Superfusion of lung cubes with Kreb's solution containing substance P or bombesin shows that ME-arg-phe can be released in a Ca++ dependent manner. The results seem to suggest that when substance P and bombesin are released by nerve impulses from their storage site in nerve fibers, they can facilitate or even trigger the release of ME-arg-phe directly or indirectly.